This invention relates to a method for brazing a magnesium-containing aluminum alloy. The aluminum alloy brazing of the present method is useful in the manufacture of a heat exchanger.
Conventionally, a chloride salt including alkaline metal and alkaline earth metal is used as a flux for the brazing of aluminum alloys. Such a chloride salt, however, is a water soluble material. Therefore, such a flux should be cleaned from the work piece by hot water, nitric acid or water immediately after the brazing operation has been completed, so that such a flux requires the use of complex cleaning operations.
Flux which does not have a character of deliquescence has become popular. Such a flux is made of potassium fluoaluminates; this flux is described in U.S. Pat. No. 3,951,238.
According to the results of an experiment conducted by the present inventors, this type flux, made of potassium fluoaluminates, can not be used in the brazing of aluminum alloys including magnesium. When the flux made of potassium fluoaluminates is used for such brazing, this flux makes the brazing process an inefficient one.
This face was found by an experiment shown some of the results of which are in FIGS. 1 and 2. The numeral 131 of FIG. 1 indicate an aluminum alloy clad by brazing material (BA12PC). The numeral 102 indicates a test piece. The test piece 102 is an aluminum alloy which includes magnesium. The weight ratios of magnesium to aluminum included in the test pieces 102 are different within various test pieces 102. Numeral 104 of FIG. 1 indicates a thin bar made of stainless steel which makes narrow gap between the aluminum alloy 101 and test piece 102.
After the test piece 102 is set on the aluminum alloy 101 and bar 104, these materials 101, 102, 103 and 104 are heated to melt the brazing material 3 clad on the aluminum alloy 1. The length L of brazing material 3 is then, measured. FIG. 3 shows results of the experiment described above. In this experiment, the length L is compared with a length M of an aluminum alloy A3003 (which does not include any magnesium).
As shown in FIG. 3, an aluminum alloy including magnesium more than 0.2% weight ratio makes the brazing inefficient. An aluminum alloy which includes about 0.8% weight ratio can not be joined by brazing. This inefficiency is, therefore, understood to be directly correlated with the relative magnitude of presence of magnesium in the aluminum alloy. The magnesium reacts with with fluoride to make magnesium fluoride, and this magnesium fluoride hinders the brazing.
The magnesium component included in the aluminum alloy is desired in order to increase the strength of the alloy. A connector of an automotive heat exchanger such as a condenser of a car air conditionor, for example, employs an aluminum alloy including magnesium for increasing its strength. Since it is desirable that the connector be of a higher strength than other parts of the condenser, the connector is made of an alloy of aluminum (Al)--zinc (Zn)--magnesium (Mg) such as the aluminum alloy A7004 (AA standard). FIG. 4 shows the relationship between the stretch strength of typical aluminum alloys and the amount of magnesium included therein. As shown in FIG. 4, such aluminum alloys having enough strength have a magnesium weight ratio of more than 1.0.